Main spindle control method

ABSTRACT

A spindle motor control method capable of high-accuracy contour machining uses a vector control processor executing a speed loop process to obtain a torque command (Tc). Vector control is performed (S4, S7) in accordance with a magnetic flux command (Φc) set at a predetermined fixed value (CFΦ) when a spindle motor is being driven in a contour control mode. This prevents irregularity in motor speed and motor vibration attributable to a delay of the actual magnetic flux of the spindle motor behind the magnetic flux command, thus enabling high-accuracy contour machining. In a normal speed control mode or an orientation mode for tool replacement, the vector control is effected (S4 to S6, S8) in accordance with the magnetic flux command (Φc), which is obtained on the basis of the torque command (Tc), its maximum value (Tcmax), maximum magnetic flux command (Φcmax) set in dependence on the rotating speed of the motor, and minimum magnetic flux (NRΦcmin, ORΦmin).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vector control method for a spindlemotor, and more particularly, to a control method for a spindle motorcapable of high-accuracy contour machining.

2. Description of the Related Art

It is known to perform vector control for speed control of a spindlemotor mounted in a machine tool, which motor is comprised of athee-phase induction motor, for instance. As shown in FIG. 2, aconventional vector control apparatus comprises a speed controller 1operable to perform proportional-plus-integral control in accordancewith a deviation between a speed command ωc and an actual speed ωor of aspindle motor (not shown) detected by a speed sensor (not shown) tothereby obtain a torque command Tc, and magnetic flux command means 2for obtaining a magnetic flux command Φc in accordance with the torquecommand Tc and the motor speed ωr.

A curve (FIG. 3) which indicates the maximum value Φcmax of the magneticflux command, represented as a function of the motor speed ωr, and theminimum value Φcmin of the magnetic flux command are set beforehand inthe magnetic flux command means 2. The maximum magnetic flux commandcurve is set so as to take a fixed value Φmax in a region where themotor speed ωr is not higher than a magnetic flux attenuation startingspeed ωp which corresponds to the DC link voltage of an inverter (notshown), and to give, in a region where the motor speed exceeds the valueωp, a maximum magnetic flux command Φcmax which is decreased as themotor speed increases. The magnetic flux command means 2 calculates thetorque command Tc in accordance with equation (1), by using the maximummagnetic flux command Φcmax obtained from the maximum magnetic fluxcommand curve in accordance wit the motor speed ωr, the torque commandTc, and the minimum magnetic flux Φcmin. As shown in FIG. 4, themagnetic flux command Φc changes in proportion to the square root of thetorque command Tc, the command being equal to the maximum magnetic fluxcommand Φcmax determined in dependence on the motor speed ωr when thetorque command takes the maximum value Tcmax, and being equal to theminimum value Φcmin when the torque command is zero.

    Φc=Tc/Tcmar .sup.1/2 ·(Φcmax-Φcmin)+Φcmin.(1)

Referring again to FIG. 2, secondary current command means 3 divides thetorque command Tc supplied from the speed controller 1 by the magneticflux command Φc supplied from the magnetic flux command means 2, therebydetermining a secondary current command I2C. Magnetic flux current means4 divides the magnetic flux command Φc by a constant k1, to determine amagnetic flux current command IO. In slip speed calculating means 5, theproduct of the secondary current command I2C and the rotor windingresistance R2 of the spindle motor is divided by the product of themagnetic flux command Φc and a constant k2, thereby obtaining a slipspeed ωs. Driving frequency command means 6 adds the moor speed ωr tothe slip frequency ωs, thereby obtaining a driving frequency ωO of thespindle motor. The vector control based on the secondary current commandI2C, the magnetic flux current command IO, and the during frequency ωOdelivered from the elements 3, 4 and 6, respectively, is executed bymeans of vector control means 7, whereby a primary current command IC isgenerated. Induced voltage estimating means 8 for respective phases(only one for one phase is shown in FIG. 2) calculate estimated inducedvoltages EO or the individual phases with a phase difference of 2π/3from one another, in accordance with a constant k3 and the magnetic fluxcommand Φc and the driving frequency ωO delivered from the elements 2and 6, respectively. In current controllers 9 for the individual phases(only one for one phase is shown), a current feedback value I_(f) foreach phase, detected by means of a current sensor (not shown), issubtracted for the primary current command Ic supplied from the element7, whereby a voltage command Vc for each phase is generated. Then, pulsewidth modulation (PWM) processing is executed in accordance with acompensated voltage command Vc, obtained by adding the estimated inducedvoltage EO supplied from the element 8 to the voltage command Vcsupplied form the element 9, and the spindle motor is driven through themedium of the inverter.

In the machine tool equipped with the vector control apparatus of FIG.2, the spindle motor is driven in any desired mode including a low-speedoperation mode, a high-speed operation mode, an orientation mode wherethe spindle is positioned for automatic tool replacement, and a controlcontrol mode (Cs contour control mode) for control machining of aworkpiece mounted on the spindle. Conventionally, the torque command Φcis calculated in accordance with equation (1), irrespective of the drivemode of the spindle motor. Meanwhile, a magnetic flux actually producedin the spindle motor is subject to a delay behind the magnetic fluxcommand Φc due to the presence of inductance. As a result, them torospeed is subject to irregularity, so that vibration occurs in the motor.In the high-speed operation mode or the orientation mode, theirregularity of the motor speed causes no substantial hindrance. In thelow-speed operation mode, however, the irregularity of the motor speedattributable to the delay of the actual magnetic flux makes it difficultto effect positioning control with required accuracy. Also in the Cscontrol control mode, which requires accuracy about 100 times as high asthe positioning accuracy for the orientation mode, control machiningwith the required accuracy sometimes cannot be achieved due to the delayof the actual magnetic flux.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a spindle motorcontrol method capable of performing highly accurate position control ofa spindle motor in a control control mode.

In order to achieve the above object, according to the presentinvention, there is provided a spindle motor control method foreffecting vector control of a spindle motor y changing a magnetic fluxcommand value in dependence on a torque command value. This controlmethod comprises the steps of: (a) determining whether or not a spindlemotor is being driven in a contour control mode where a workpiecemounted on a spindle is subjected to contour machining while the spindlemotor is being operated at low speed; and (b) fixing the magnetic fluxcommand value to a predetermined fixed value during the contour controlmode.

Since the magnetic flux command value is fixed to the predeterminedfixed value during the contour control mode, as described above, theactual magnetic flux never changes even when the torque command changes,so that the spindle motor speed can be prevented from becoming irregulardue to a change of the actual magnetic flux. Thus, the position controlof the spindle motor can be highly accurately effected in the contourcontrol mode, so that the contour machining can be executed with highaccuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing a magnetic flux calculation process in aspindle motor control method according to one embodiment of the presentinvention;

FIG. 2 is a block diagram showing a vector control apparatus;

FIG. 3 is graph showing a maximum magnetic flux command curve used in aconventional magnetic flux calculation process; and

FIG. 4 is a diagram for illustrating the conventional magnetic fluxcalculation process

DESCRIPTION OF THE PREFERRED EMBODIMENT

A spindle motor control method according to the present invention isembodied by a vector control apparatus (not shown), for instance, whichincludes a processor operable to fulfill, by means of softwareprocessing, the respective functions of various elements shown in FIG.2. The vector control apparatus, which is mounted to, e.g., a machinetool (not shown), is arranged to drive a spindle of the machine tool inany drive mode including a normal speed control mode, an orientationmode, and a control control mode (Cs contour control mode), inaccordance with a machining program, and load a built-in registerthereof with code information which is indicative of the current drivemode specified by the machining program.

The processor for vector control is operable to execute a speed loopprocess at intervals of a predetermined period to thereby obtain atorque command Tc corresponding to the output of the speed controller 1of FIG. 2, and to execute a magnetic flux calculation process of FIG. 1in each processing period.

More specifically, the processor determines whether or not an actualmotor speed ωr detected by a speed sensor (not shown), such as anencoder, is equal to or less than a magnetic flux attenuation startingspeed ωp (Step S1). If the motor speed ωr is not higher than the valueωp, the maximum value Φcmax of a magnetic flux command is set at apredetermined value Φmax (Step S2). If the speed ωr exceeds the valueωp, the product of the magnetic flux attenuation starting speed ωp andthe predetermined magnetic flux command value Φmax is divided by themotor speed ωr to calculate the maximum magnetic flux command Φcmax(Step S3), as indicated by equation (2).

    Φcmax=ωp·Φmax/ωr.             (2)

Subsequently, the processor determines whether the current spindle motordrive mode is the normal speed control mode, or the orientation mode, orthe Cs control control mode (Step S4). If the drive mode is the normalspeed control mode or the orientation mode, the minimum value NRΦcmin orORΦcmin of the command magnetic flux for this mode is loaded into aregister A (Step S5 aor S6). Then, the magnetic flux command Φc iscalculated in accordance with equation (1), by using the minimummagnetic flux NRΦcmin orORΦcmin, the torque command Tc obtained in thespeed loop process, the maximum magnetic flux command Φcmax obtained inStep S2 or S3, and the maximum value Tcmax of the torque command.Whereupon, conventional vector control is executed in accordance withmagnetic flux command Φc.

If it is concluded in Step S4 that the current drive mode is the Cscontour control mode, on the other hand, the processor sets the magneticflux command Φc at a predetermined fixed value CFΦ for the Cs contourcontrol mode, and then performs vector control in accordance with thispredetermine value CFΦ. during the vector control, the actual magneticflux never changes, and a delay of the actual magnetic flux relative tothe magnetic flux command Φc(=CFΦ) is not caused. As a result, thespindle motor suffers neither irregularity in speed nor vibration, sothat contour matching can be defected with high accuracy.

What is claimed is:
 1. A spindle motor control method using vectorcontrol of a spindle motor, comprising the steps of:(a) determiningwhether a spindle motor is in a contour control mode where a workpiecemounted on a spindle motor is operated at a low speed; (b) setting amagnetic flux command value to a predetermined fixed value during thecontour control mode; (c) setting the magnetic flux command value independence on the torque command value when said determining in sep (a)determines that the spindle motor is not operating in the contourcontrol mode; and (d) controlling the spindle motor in dependence uponthe magnetic flux command set in one of steps (b) and (c).